OSI Model (7 Layers)
| Layer | Name | Purpose | Examples |
|---|---|---|---|
| 7 | Application | User‑level network services | HTTP, DNS, SMTP, FTP |
| 6 | Presentation | Encoding, encryption, compression | SSL/TLS, MIME |
| 5 | Session | Connection management | RPC, NetBIOS, PPTP |
| 4 | Transport | End‑to‑end delivery | TCP, UDP |
| 3 | Network | Routing & addressing | IP, ICMP, IGMP |
| 2 | Data Link | MAC addressing, framing | Ethernet, PPP, 802.11 |
| 1 | Physical | Bits on the wire | Copper, fiber, RF |
TCP/IP Model (4 Layers)
| Layer | Purpose | Examples |
|---|---|---|
| Application | Protocols & services | HTTP, DNS, SSH, DHCP |
| Transport | Segments/datagrams | TCP, UDP, QUIC |
| Internet | Routing | IPv4, IPv6, ICMP |
| Network Access | Physical + link | Ethernet, Wi‑Fi, PPP |
Common Port Numbers
Well‑Known Ports
| Port | Protocol | Description |
|---|---|---|
| 20/21 | FTP | File Transfer |
| 22 | SSH | Secure shell |
| 23 | Telnet | Remote terminal (insecure) |
| 25 | SMTP | Email sending |
| 53 | DNS | Name resolution |
| 67/68 | DHCP | IP assignment |
| 69 | TFTP | Simple file transfer |
| 80 | HTTP | Web traffic |
| 110 | POP3 | Email retrieval |
| 123 | NTP | Time sync |
| 143 | IMAP | Email retrieval |
| 161/162 | SNMP | Network management |
| 389 | LDAP | Directory services |
| 443 | HTTPS | Secure web |
| 514 | Syslog | Logging |
| 587 | SMTP (submission) | Authenticated email |
| 636 | LDAPS | Secure LDAP |
| 993/995 | IMAPS/POP3S | Secure email |
Subnet Masks & CIDR
| CIDR | Mask | Hosts |
|---|---|---|
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /28 | 255.255.255.240 | 14 |
| /29 | 255.255.255.248 | 6 |
| /30 | 255.255.255.252 | 2 |
| /32 | Single host | 1 |
Ethernet Standards (IEEE 802.3)
| Standard | Speed | Medium | Max Distance |
|---|---|---|---|
| 10BASE‑T | 10 Mbps | Cat3+ | 100 m |
| 100BASE‑TX | 100 Mbps | Cat5+ | 100 m |
| 1000BASE‑T | 1 Gbps | Cat5e+ | 100 m |
| 1000BASE‑SX | 1 Gbps | MMF | 220–550 m |
| 1000BASE‑LX | 1 Gbps | SMF | 5–10 km |
| 10GBASE‑T | 10 Gbps | Cat6a+ | 100 m |
| 10GBASE‑SR | 10 Gbps | MMF | 26–400 m |
| 10GBASE‑LR | 10 Gbps | SMF | 10 km |
| 10GBASE‑ER | 10 Gbps | SMF | 40 km |
| 40GBASE‑T | 40 Gbps | Cat8 | 30 m |
| 100GBASE‑LR4 | 100 Gbps | SMF | 10 km |
| 100GBASE‑ER4 | 100 Gbps | SMF | 40 km |
Cable Types & Max Distances
Twisted Pair
| Category | Max Speed | Max Distance |
|---|---|---|
| Cat5e | 1 Gbps | 100 m |
| Cat6 | 1–10 Gbps | 55 m (10G) |
| Cat6a | 10 Gbps | 100 m |
| Cat7/7a | 10–40 Gbps | 100 m |
| Cat8 | 25–40 Gbps | 30 m |
Fiber
| Type | Mode | Distance |
|---|---|---|
| OM1 | MMF | 275–550 m |
| OM2 | MMF | 550 m |
| OM3 | MMF | 300 m (10G) |
| OM4 | MMF | 400 m (10G) |
| OS1/OS2 | SMF | 10–40+ km |
Fiber Connector Types
| Connector | Type | Notes |
|---|---|---|
| LC | Duplex | Most common modern connector |
| SC | Duplex | Older but still widely used |
| ST | Single | Legacy twist‑lock |
| MPO/MTP | Multi‑fiber | Used for 40G/100G |
Wireless Standards (802.11)
| Standard | Frequency | Max Speed |
|---|---|---|
| 802.11b | 2.4 GHz | 11 Mbps |
| 802.11g | 2.4 GHz | 54 Mbps |
| 802.11n | 2.4/5 GHz | 600 Mbps |
| 802.11ac | 5 GHz | 1.3 Gbps+ |
| 802.11ax (Wi‑Fi 6) | 2.4/5 GHz | 9.6 Gbps |
| 802.11be (Wi‑Fi 7) | 2.4/5/6 GHz | 30+ Gbps |
Common Networking Commands
Windows
ipconfig /all— full IP configurationping <host>— connectivity testtracert <host>— route pathnetstat -ano— active connectionsnslookup <domain>— DNS lookup
Linux / macOS
ifconfigorip a— interface infoping— connectivitytraceroute— route pathnetstat -tulpn— listening portsdig <domain>— DNS lookup
Troubleshooting (Quick Reference)
- Physical Layer
- Check cables
- Check link lights
- Replace patch cable
- IP Layer
- Check IP address
- Check gateway
- Ping local gateway
- DNS Layer
- Test DNS resolution
- Try alternate DNS
- Routing Layer
- Check routing table
- Trace route to destination
- Application Layer
- Test service port
- Check firewall rules